Gamma-ray Bursters cross the 'Line of Death'

Gamma-ray Bursters cross the 'Line of Death'

Fireballs and gamma-ray bursts are not the same thing

October 13, 1998: Cosmic gamma-ray bursts have been called
the greatest mystery of modern astronomy. They are powerful blasts
of gamma- and X-radiation that come from all parts of the sky,
but never from the same direction twice. Space satellites indicate
that Earth is illuminated by 2 to 3 bursts every day.

What are they? No one is certain. Until recently we didn't even
know if they came from the neighborhood of our own solar system
or perhaps from as far away as the edge of the universe. The
first vital clues began to emerge in 1997 when astronomers detected
an optical counterpart to a gamma-ray burst. In February 1997
the BeppoSAX X-ray astronomy satellite pinpointed the position
of a burst in Orion to within a few arcminutes. That allowed
astronomers to photograph the burst, and what they saw surprised
them. They detected a rapidly fading star, probably the aftermath
of a gigantic explosion, next to a faint amorphous blob believed
to be a very distant galaxy.
The Burst and Transient Source Experiment (BATSE)
aboard the Compton Gamma Ray Observatory, pictured above, has
recorded over 2000 cosmic gamma-ray bursts since it began operations
in 1991.

This image from the
Hubble Space Telescope shows the optical afterglow from a gamma-ray
burst detected in February 1997. The bright spot is thought to
be an expanding fireball, and the weak diffuse emission (below
and to the right) may be the distant host galaxy.

Since then seven more optical counterparts have been discovered.
A recent discovery makes gamma-ray bursts seem more fantastic
than ever. Shri Kulkarni of Caltech and his colleagues found
that a gamma-ray burst recorded in December 1997 came from a
faint galaxy with a redshift of 3.4. That means that the burst
originated over 12 billion light years away. Kulkarni noted that
"The energy released by this burst in its first few seconds
staggers the imagination." Indeed, it was one of the biggest
explosions since the Big Bang itself.

Now that we know where gamma-ray bursts come from -- very
far away -- the next daunting task is to understand what
causes them. In the absence of much hard data theorists have
proposed a multitude of possible scenarios, from super-supernovae
to mutually annihilating neutron stars. It is widely thought
that the x-ray, optical, and radio afterglows might provide some
clues.
The light curves of the few known optical and X-ray counterparts
are consistent with that of an expanding fireball that is glowing
because of a "Synchrotron Shock". The basic idea is
that a tremendous explosion ejects a shock wave of material that
accelerates charged particles, like electrons and protons, to
velocities near the speed of light. Dr. Robert Preece, a gamma-ray
astrophysicist at the University of Alabama in Huntsville, likened
the shock wave to a wave on the beach. "A shock forms when
the wave crest starts to fall over, and scud from the wave shoots
out ahead."

A synchrotron shock
wave can be visualized as an ocean wave. A shock forms when the
wave crest starts to fall over, and scud from the wave shoots
out ahead. In the cosmic shock wave, the 'scud' is composed of
charged particles that give rise to synchrotron radiation.

In the cosmic shock wave, the 'scud' is electrons and protons.
They accelerate ahead of the wave and spiral around magnetic
fields lines, producing a form of radiation called synchrotron
emission. Synchrotron emission is seen all the time here on Earth
as a blue glow in particle accelerators, and radio astronomers
detect it coming from the Milky Way.
Fortunately, the Synchrotron
Shock Model makes a testable prediction. The spectrum of a typical
gamma-ray burst looks like the plot at the right. At lower energies,
i.e., less than a few hundred keV (kilo-electron volts), the
spectrum is fairly flat. A nearly horizontal line fits that part
of the spectrum fairly well. At higher energies the spectrum
is steeper. The Synchrotron Shock Model predicts that the slope
of the line that fits the lower energy part of the spectrum cannot
be greater than -2/3.

In a recently published edition of the Astrophysical Journal
Letters Rob Preece and his collaborators from the University
of Alabama examined over 100 bright bursts collected by the BATSE
instrument on the Compton Gamma Ray Observatory and measured
the slopes of their low-energy spectra. The figure at left shows
their data. They plotted the
slope of the low energy part of the spectra (vertical axis) vs.
the peak burst energy (horizontal axis). The red line is the
so-called "Line of Death", corresponding to a spectral
slope greater than -2/3. If a data point falls above the line,
that gamma-ray burst cannot have been caused by a synchrotron
shock, and thus the Synchrotron Shock Model is "dead"
for that burst. Preece et al. found that 44% of the bursts fell
above the Line of Death. If we assume that all gamma-ray bursts
are caused by the same thing, this means that none can be due
to a synchrotron shock.

Thanks to their work we now know another thing that gamma-ray
bursts are not. They can't be caused by a synchrotron
shock. Interestingly though, there is strengthening evidence
that the optical counterparts, the glow from fireballs that appear
to be the aftermath of gamma-ray bursts in distant galaxies,
are caused by synchrotron shock waves. Whatever makes
the fireball glow is apparently different from the mechanism
that makes gamma-ray bursts. It's yet another mystery in the
fantastic saga of gamma-ray bursters.
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